Gas turbine engines for use in an aircraft are well known in the art. Generally, gas turbine engines are formed by a fan section, a compressor section, a combustor section, and a turbine section. A primary flow path for working medium gases extends serially through these sections. A secondary flow path for working medium gases extends rearwardly, radially outward of the primary flow path.
The compressor section includes an upstream low pressure compressor and a downstream high pressure compressor. Both compressors are formed by a stator assembly and a rotor assembly. The rotor assembly has a plurality of airfoils or blades which extend radially outward from a rotor disk. Each blade generally has a root region, an opposed tip, and a midspan region disposed therebetween. The midspan region includes an upstream leading edge extending from the root region to the tip, a downstream trailing edge extending from the root region to the tip and spaced from the leading edge and pressure and suction surfaces extending between the leading and trailing edges.
In operation, the working medium gases flow along the primary flow path and enter the compressor section where the gases are compressed. These compressed gases are burned in the combustor section and discharged to the turbine section. The turbine section extracts work from the gases to power the compressor and fan sections. The fan section draws in the working medium gases along the flow paths and raises the pressure of the gases to produce thrust.
Particularly during take off and landing, as the fan section draws in the working medium gases it also sucks in sand. The sand abrades the blades as it moves past them causing the blades to erode. Therefore, it is industry practice to check the blades for erosion damage during comprehensive maintenance operations. The time between comprehensive maintenance operations is designated as a full run of service life. A stub run of service life is half a full run. The number of hours in a full run of service life varies according to the engine's operating conditions.
For blades that erode at the trailing edge before they erode at the leading edge, the blade erosion check includes several procedures. First, an inspector visually inspects the contour of blade. If the inspector determines that the contour toward the trailing edge is too sharp the blade must be scrapped (i.e., cannot be returned to service). Second, the inspector measures each blades' chord length. The chord length is the straight line distance between the leading edge and the trailing edge. If the inspector determines that the chord length for the blade is less than a minimum chord length the blade must be scrapped. Third, on some engines, the inspector measures each blades' thickness at least at one point, which is a predetermined distance from the trailing edge and the blade tip. If the inspector determines that the thickness at that point is less than a minimum blade thickness, the blade must be scrapped.
There are several problems with the aforementioned procedures. First, visually inspecting the contour of the blade for a contour that is too sharp allows too much subjectivity to enter into the measurement. This subjectivity leads to inconsistency in results from inspector to inspector depending on a number of factors, such as the inspector's experience.
Second, measuring the chord length only allows the inspector to decide whether the blade should or should not be returned to service. In certain engines once a blade wears at the trailing edge and the chord length diminishes beyond a critical chord length, the blade deteriorates more rapidly than before. Thus, if this type of blade is returned to service because its chord length is greater than the minimum chord length, and during service the blade erodes at the trailing edge to the critical chord length the blade may deteriorate quickly thereafter and engine performance will decrease. Since the blade was returned to service without a time limit, the poorly performing blade will remain in the engine until the next comprehensive maintenance operation. Inspectors usually do not scrap blades that have a chord length close to the critical chord length, because good blades may be scrapped accidentally and the inspector who must justify every decision to scrap a blade, may have a difficult time justifying scrapping a blade that passed the blade erosion check procedures.
Third, since the trailing edge on each blade eroding differently, the inspector never measures the blade thickness at the same point, which is measured from the trailing edge. This leads to inconsistency in measurements from blade to blade. Also measuring at a point could lead to an incorrect decision to return or not to return a blade to service because an anomaly in the blade width that does not indicate erosion, but a manufacturing or operating aberration could be misinterpreted. Although the effects of an anomaly in the blade width are minimized by taking the thickness measurement at two points which some inspectors do according to procedure, misinterpretation can still occur.
Lastly, the current check cannot be performed on the blades until they are removed from the rotor assembly. Since there are numerous blades requiring inspection, removal of the blades is labor intensive and time consuming. If the blade does not have significant erosion damage, its removal for inspection is inefficient.
More accurately determining the service life remaining in a blade will allow blades that previously would have been scrapped to be return to service. Determining the amount of service life remaining in a blade will allow only those blades, which are in adequate condition to remain in service for a significant time without significant reduction in performance to be returned to service. A method and a gauge which does not require removing all the blades from the rotor assembly before checking them, thus requiring only those blades that need to be replaced, to be removed would save time on maintenance. All the aforementioned improvements would result in significant cost savings. Therefore, scientists and engineers have been searching for an inexpensive, repeatable, and efficient method and a gauge for measuring the service life remaining in a blade.